RFC: Add a mode to not generate debug! statements, use by default for core/std

[pcwalton]

I just looked at the source for any pipes code and a huge amount of debug! stuff in the pipes code is getting monomorphized, inlined, and written into every single send and recv in all user code. I propose disabling generation of debug! by default; it's purely a hazard for core and std.

[nikomatsakis]

This seems sensible. I have sometimes wondered whether removing debug! makes any difference in the performance of rustc, as well. I would hate to have to remove them by hand---I personally prefer debug! statements to comments as a way of documenting what's happening. It means that when a bug occurs I can turn them on and follow along.

[catamorphism]

Seems reasonable to me.

[bstrie]

Nominating for Maturity 5.

[graydon]

subset of #3309, declining

[emberian]

I "disabled" this once by having debug!() expand into nothing, and it was a ~second speedup for rustc when RUST_LOG isn't set. It's just an extra branch for every debug! iirc.

This is fairly easy, check for some sort of argument, like --debug off, and use a different core-macros that has an empty expansion of debug!. Hacky, but effective. Could also do it for the other logging levels, though not as useful.

[huonw]

Implemented in b48e37e.